Low-EMI circuit board and low-EMI cable connector

ABSTRACT

A low-EMI cable connector mounted on a transmission cable for connecting circuits comprises n (n: 1, 2, . . . ) cylindrical members having arranged on the inner surface of a dielectric portion covering the whole periphery of the transmission cable. A short-circuiting member covering the whole periphery of the transmission cable is formed on the termination side of the cylindrical members thereby to form a short-circuiting termination line. The resonance frequency of the short-circuiting termination line is set equal to the resonance frequency of the transmission cable.

TECHNICAL FIELD

The present invention relates to a low-EMI circuit board on whichcircuit elements such as an IC and a LSI are mounted, or more inparticular to a low-EMI cable connector for suppressing the unrequiredradiation in the low-EMI circuit board and the transmission cable insuch a manner as to suppress the radiation mainly of differential modefrom the mounted parts.

BACKGROUND ART

Conventionally, in a multilayer circuit board having arranged thereinsignal lines, a power line and a ground line and having arranged thesurface thereof an IC elements, a LSI element and circuits, a seriousproblem is posed by the fact that with the increase in speed anddensity, the unrequired radiation is liable to occur due to highharmonics which have an effect on other devices.

The unrequired radiation is roughly divided into two types, thecommon-mode radiation caused by the resonance due to the potentialfluctuations of the power layer and the ground layer and the radiationof differential mode caused by the signal line layers and the componentparts mounted. In the prior art, various methods have been proposed toreduce these unrequired radiation.

A method generally employed for reducing the radiation of differentialmode is by shielding, and a method specifically employed is by coating aconductive paste containing a resistance material on the surface of theboard.

In order to send a signal to the circuit board from an external source,a transmission line such as a coaxial cable is connected by a connectorwith an external signal source. Such a connection is schematically shownin FIG. 13.

In this diagram, the signal source is designated as a transmittingterminal unit 100 and the circuit board receiving signals from thetransmitting terminal unit 100 is designated as a receiving terminalunit 101, with a coaxial cable 102 connected between them. The circuitboard constituting the receiving terminal unit 101 is connected to thecoaxial cable 102 by a connector not shown. The transmitting terminalunit 100 is also connected with the coaxial cable 102 by a connector notshown.

In the transmitting terminal unit 100, an outward line 100 a connectedto a signal source 100 c for generating a pulse-like signal of frequencyωj and voltage V0 is connected to an internal conductor 102 a of thecoaxial cable 102, and an inward line 100 b is connected to an externalconductor 102 b of the coaxial cable 102, each by a connector not shown.Also, the receiving terminal unit 101 is equivalently expressed by areceiving line 110 a, a return line 101 b and a load impedance ZLconnected between them. This receiving line 100 a is connected to theinternal conductor 102 a of the coaxial cable 102, and the inward line101 b is connected to the external conductor 102 b of the coaxial cable102, respectively, by a connector not shown. The inward line 100 b ofthe transmitting terminal unit 100 and the return line 101 b in thereceiving terminal unit 101 are grounded, and the external conductor 102b of the coaxial cable 102 is used as a grounding line.

In this configuration, a signal line is formed of the outward line 100 afrom the signal source 100 c of the transmitting terminal unit 100, theinternal conductor 102 a of the coaxial cable 102, the receiving line101 a, the load resistor R, the return line 101 b of the receivingterminal unit 101, the external conductor 102 b of the coaxial cable 102and the inward line 100 b of the transmitting terminal unit 100.

In this signal line, the signal output from the signal source 100 c inthe transmitting terminal unit 100 is sent to the internal conductor 102a of the coaxial cable 102 as a voltage V1 a and a current i1 a, andreceived at the receiving terminal unit 101 as a voltage V1 b and acurrent ilb, respectively. Also, in the return path of this signal line,a signal of a voltage V2 b and a current i2 b flows from the receivingterminal unit 101 along the inner surface of the external conductor 102b of the coaxial cable 102. Not only that, the current is reflected byan equivalent impedance at a junction point B between the coaxial cable102 and the receiving terminal unit 101, so that the current leaks outto the outer surface of the external conductor 102 b of the coaxialcable 102. This flows as a leakage current i3 b along the outer surfaceof the external conductor 102 b of the coaxial cable 102. The signalflowing along the inner surface of the external conductor 102 b is inputto the transmitting terminal unit 100 as a voltage V2 a and a current i2a. The current is also reflected by an equivalent impedance at ajunction point A between the coaxial cable 102 and the transmittingterminal unit 100. As a result, part of the current i2 a leaks out tothe outer surface of the external conductor 102 b of the coaxial cable102 and flows along the outer surface of the external conductor 102 b ofthe coaxial cable 102 as a leakage current i3 a.

The coaxial cable 102 forming this signal line has a resonance point ofa wavelength λ satisfying the relation L=(2n−1)·λ/4 (n: a positiveinteger) where L is the length of the coaxial cable 102. Therefore, aslong as the wavelength of the currents i3 a, i3 b flowing along theouter surface of the external conductor 102 b of the coaxial cable 102is sufficiently away from the wavelength λ, the currents i3 a, i3 bwhich are originally very small pose no problem. In the case where thewavelength of the currents i3 a, i3 b is proximate to the resonancepoint of the coaxial cable 102, however, the coaxial cable 102 developsa resonance, operates as a mono-pole antenna, and thus generates anunrequired electromagnetic radiation. Let the length L of the coaxialcable 102 be 1 m, for example. A resonance point occurs at a resonancepoint of frequency equivalent to odd multiples of f=3×108/4×1 =75 MHz.

The leakage current described above could be eliminated, if the case ofthe interior of the transmitting terminal unit 100, the interior of thecoaxial cable 102 and the interior of the case of the receiving terminalunit 101 could be completely hermetically closed by integrating the caseof the transmitting terminal unit 100 completely with the outer surfaceof the external conductor 102 b of the coaxial cable 102 and also byintegrating the case of the receiving terminal unit 101 completely withthe outer surface of the external conductor 102 b of the coaxial cable102. Actually, however, such a configuration is substantially impossibleto realize. Therefore, the occurrence of the unrequired radiationdescribed above is unavoidable.

In view of this, according to the prior art, in order to suppress theunrequired radiation, a ferrite core 103 a called a common mode core ora common mode choke is arranged on the side end of the transmittingterminal unit 100 of the coaxial cable 102, and in similar manner, aferrite core 103 b is arranged on the side end of the receiving terminalunit 101.

The provision of the ferrite cores 103 a, 103 b is equivalent to theinsertion of a series circuit including an inductance and a resistor inthe signal line along the outer surface of the external conductor 102 bof the coaxial cable 102 due to the inductance and the polarizationderived from the ferrite cores 103 a, 103 b. It follows, therefore, thatthe leakage currents i3 a, i3 b flowing along the same outer surface aresuppressed. The absolute value of the impedance of the ferrite cores 103a, 103 b is conventionally set to about 100Ω from the viewpoint of thematerial and structure.

According to the above-mentioned conventional method, however, aconductive paste is coated on a comparatively flat portion of thesurface of the board but cannot be coated on the component parts mountedor the portion where they are mounted. Even in the case where the boardsurface is shielded by the conductive paste, therefore, the shield layeris opened in the portion where the component parts are mounted, and theunrequired radiation leaks out from the opening and the unrequiredradiation (common mode radiation) occurs anew due to the resonance atthe opening. The unrequired radiation thus cannot be suppressedsufficiently.

Also, the unrequired radiation from the transmission line such as acoaxial cable connected to the board, as explained with reference toFIG. 13, can be suppressed to some degree, but not necessarily to asufficient degree, by arranging a ferrite core at the ends of thetransmission line. In the foregoing description with reference to FIG.13, the absolute value of the impedance of the ferrite cores 103 a, 103b is set to 100 Ω. If 100 Ω is not sufficient, however, the absolutevalue of the impedance is increased by arranging a plurality of ferritecores 103 a and ferrite cores 103 b. In this way, the effect ofsuppressing the leakage currents i3 a, i3 b can further be increased atthe sacrifice of the requirement of using a bulky, heavy ferrite core.The use of a plurality of ferrite, on the other hand, is equivalent tothe coils wound in a plurality of turns, between which theelectro-static capacitance may occur, thereby posing the problem of theoccurrence a new resonance.

A more critical problem is that even when a quality ferrite material isused for the ferrite cores 103 a, 103 b, the permeability μ thereof hassuch a frequency characteristic that the frequency of 300 MHz or highersharply reduces the permeability μ and makes it impossible to produce asufficiently large impedance. At such a frequency, the ferrite cores 103a, 103 b have a lesser effect of reducing the leakage currents i3 a, i3b, and are unable to suppress the unrequired radiation generated fromthe coaxial cable 102.

In the case where the clock frequency of the signal involved is as lowas about 10 MHz, for example, it is sufficiently lower than 300 MHz andtherefore it is possible to use the ferrite cores 103 a, 103 b with asufficiently large permeability μ. Thus, the fundamental wave and tripleharmonics of the problem leakage currents i3 a, i3 b causing theunrequired radiation can be sufficiently suppressed, and the frequencycharacteristic of the permeability μ is not a serious problem. In recentyears, however, the clock frequency of the personal computer or the likehas further increased to not less than 100 MHz or not less than 200 MHz,etc. With this high clock frequency, the permeability μ of the ferritecores 103 a, 103 b is decreased to such an extent, at the fundamentalwave and the triple harmonics of the signal, that the effect of reducingthe unrequired radiation cannot be exhibited.

An object of the present invention is to provide a low-EMI circuit boardwhich obviates the above-mentioned problems and is capable ofeffectively suppressing the radiation of mainly the differential mode.

Another object of the invention is to provide a low-EMI cable connectorwhich is both compact and simple in configuration and can effectivelysuppress the unrequired radiation in the signal transmission line.

DISCLOSURE OF INVENTION

In order to achieve the objects mentioned above, in a low-EMI circuitboard according to the present invention, the whole surface of the boardincluding the component parts mounted thereon is covered with a shieldplate, which is electrically connected to the ground layer with theshield plate inserted therein. As a result, the unrequired radiation ofdifferential mode generated from the component parts mounted and thesignal lines is contained between the shield plate and the ground layerand cannot leak out.

Also, the low-EMI circuit board according to the present inventioncomprises a loss layer on at least one surface of the conduction layerof the shield plate. With the containment of the unrequired radiation ofdifferential mode, a high frequency current flows in the short-circuitloop formed by the shield plate and the ground layer. Then, theresulting resonance causes the unrequired external radiation (commonmode radiation). This loss layer attenuates the current and can suppressthe unrequired radiation.

Further, in the low-EMI circuit board according to this invention, theportion of the shield plate around the board is connected at multiplepoints to the ground layer. As a result, the resonance frequency of theloop including the shield plate and the ground layer can be transferredto a high frequency band higher than the frequency region to besuppressed. Thus it is possible to sufficiently suppress the common-moderadiation from the board side with the differential-mode radiation as anoise source.

Further, in the low-EMI circuit board according to this invention, theportion of the shield plate around the board is connected to the groundlayer through a matching termination resistor thereby to suppress thepotential fluctuations. As a result, the common-mode radiation from theboard side is suppressed not to leak out.

Further, in the low-EMI circuit board according to the presentinvention, the shield plate is connected at multiple points to theground layer around the component parts mounted on the board such as theLSI element and the drive IC element which are operated at high speed.The fast-operating component parts mounted on the board are liable todevelop the differential-mode radiation. By connecting the shield plateto the ground layer at multiple points around the component partsmounted, these component parts are individually shielded. Thus, theradiation of differential mode from these component parts mounted isreduced. Further, in the portion around the board where the shield plateis connected to the ground layer at multiple points, it follows that theelectrical connection structure is doubled for shielding. The shieldingeffect thus becomes conspicuous.

In order to achieve the second object described above, a low-EMI cableconnector according to this invention comprises n (n: 1, 2, . . . )cylindrical members arranged on the inner surface of a dielectricportion surrounding the whole periphery of a transmission cable, whereina short-circuiting member surrounding the whole periphery of thetransmission cable is arranged on the termination side of thecylindrical members thereby to form a short-circuiting termination line,and the resonance frequency of the short-circuiting termination line isconfigured to equal the resonance frequency of the transmission cable.

Also, in the low-EMI cable connector according to this invention, thelength li of the i-th (i: 1, 2, . . . , n) one of the cylindricalmembers forming the short-circuiting termination line is given as$\begin{matrix}{1_{t} = {\frac{1}{4} \times \frac{\lambda_{i}}{\sqrt{ɛ_{ri}}}}} & \left( {{Expression}\quad 1} \right)\end{matrix}$

where

λi=c/fi

c=velocity of light

fi=i-th fundamental resonance frequency of transmission cable

εri=dielectric constant of the dielectric portion of the i-thcylindrical member

Further, in the low-EMI cable connector according to this invention, aplurality of the cylindrical members described above forming ashort-circuiting termination line are arranged coaxially.

Furthermore, the low-EMI cable connector according to the invention ischaracterized in that the short-circuiting member is configuredreplaceably.

What is more, in the low-EMI cable connector according to thisinvention, a plurality of the cylindrical members forming theshort-circuiting termination line share a center axis and are arrangedalong the center axis.

In addition, in the low-EMI cable connector according to this invention,the cylindrical members forming the short-circuiting termination lineeach can be adjusted in the direction of the center axis.

With this configuration, the impedance of the short-circuitingtermination line becomes substantially infinitely large at the resonancefrequency thereof, and the current of the resonance frequency flowing inthis line is suppressed to almost zero. In view of this, the currentcausing the unrequired radiation can be effectively suppressed bysetting the resonance frequency of this short-circuiting terminationline equal to the frequency generating the unrequired radiation by theresonance of the transmission cable.

Assume that the short-circuiting termination line is configured of abottomed cylindrical portion including a dielectric portion. Let εri bethe dielectric constant of the dielectric portion, and λi be thewavelength of the current flowing in the conductor portion. In thisshort-circuiting termination line, the wavelength λi′ is given asλi/{square root over ( )}εri. Thus, the length of the bottomedcylindrical portion of the short-circuiting termination line can berendered as short as 1/{square root over ( )}ri times the length of thetransmission cable. When a dielectric material having εri of 900 isused, for example, the length of the bottomed cylindrical portion can beas long as {fraction (1/30)} of the transmission cable involved. For thecoaxial cable 75 cm long which resonates at 100 MHz, for example, thelength of the bottomed cylindrical portion is only 2.5 cm.

In the case where the transmission cable has a different fundamentalresonance frequency as when the external conductor of the coaxial cableis grounded midway or otherwise, an unrequired radiation correspondingto the fundamental frequency occurs. By arranging a plurality of theshort-circuiting termination lines coaxially or along the center axis,however, the current for generating the unrequired radiation for eachfundamental resonance frequency can be effectively suppressed by eachshort-circuiting termination line.

In order to achieve the above-mentioned second object, the low-EMI cableconnector according to this invention is cylindrical and comprises, onthe inner surface thereof, a dielectric portion surrounding the wholeperiphery of the transmission cable, and a resistor making up a matchingtermination resistor on the termination side of the cylindrical member.

With this configuration, the presence of the matching terminationresistor causes a current to flow in the dielectric portion withoutreflection, and this current is effectively suppressed by beingthermally converted by the matching termination resistor. In this case,the length of the cylindrical member is arbitrary, the above-mentionedmatching termination resistor compatible with the cylindrical member isprovided.

In order to achieve the above-mentioned second object, the low-EMI cableconnector according to this invention comprises n (n: 1, 2, . . . )cylindrical members with a dielectric portion on the inner surfacearranged thereof and surrounding the whole periphery of the transmissioncable, wherein the termination side of the cylindrical members is openand forms an open termination line, and the resonance frequency of theopen termination line is equal to the resonance frequency of the trans-mission cable.

Also, in the low-EMI cable connector according to this invention, thelength of the i-th (i: 1, 2, . . . , n) one of the cylindrical membersforming the open termination line is given as $\begin{matrix}{\frac{1_{t}}{2} = {\frac{1}{4} \times \frac{\lambda_{i}}{\sqrt{ɛ_{ri}}}}} & \left( {{Expression}\quad 2} \right)\end{matrix}$

where

λi=c/fi

c=velocity of light

fi=i-th fundamental resonance frequency of the transmission cable

εri=dielectric constant of dielectric portion of the i-th cylindricalmember

Further, in the low-EMI cable connector according to this invention, aplurality of the cylindrical members forming the open termination lineshare a center axis and are arranged along the center axis.

Furthermore, the low-EMI cable connector according to this invention isconfigured in such a manner that the cylindrical members forming theopen termination line each is capable of being adjusted in positionalong the direction of the center line.

This configuration is equivalent to the arrangement in which theshort-circuiting termination is provided at the intermediate position ofthe open termination line, i.e. at the intermediate position of thecylindrical member at the resonance frequency. Under this condition, theimpedance becomes almost infinitely large. Consequently, by setting theresonance frequency equal to the resonance frequency causing theunrequired radiation of the transmission cable, the current of thefrequency causing the unrequired radiation of the transmission cable canbe effectively suppressed.

In this case, however, the length of the open termination line, i.e. thelength of the cylindrical member is double the length for the inventionhaving a short-circuiting termination line. Nevertheless, thecylindrical portion is reduced in size.

Also, in the case where the transmission cable has a differentfundamental resonance frequency and generates the unrequired radiationcorresponding to the fundamental frequency like in the invention havingthe short-circuiting termination line described above, the currentcausing the unrequired radiation for each fundamental resonancefrequency can be effectively suppressed with each open termination lineby arranging a plurality of the open termination lines coaxially oralong the center axis.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view showing a low-EMI circuit board according toan embodiment of the present invention,

FIG. 2(a) and FIG.(b) are a diagram showing the position at which theloss layers are formed in the power layer and the ground layer in FIG.1,

FIG. 3 is a sectional view showing a low-EMI circuit board according toan embodiment of the present invention,

FIG. 4(a) and FIG. 4(b) are a sectional view showing a low-EMI cableconnector according to an embodiment of the present invention,

FIG. 5 is an impedance characteristic diagram of the low-EMI cableconnector shown in FIG. 4,

FIG. 6 is a sectional view showing the mounting and the operation on thetransmitting unit side of the low-EMI cable connector shown in FIG. 4,

FIG. 7 is a circuit diagram showing an equivalent circuit mounted asshown in FIG. 6,

FIG. 8 is a sectional view showing the mounting and the operation on thelow-EMI circuit board side of the low-EMI cable connector shown in FIG.4,

FIG. 9 is a sectional view showing a low-EMI cable connector accordingto another embodiment of the present invention,

FIG. 10 is a sectional view showing a low-EMI cable connector accordingto still another embodiment of the present invention,

FIG. 11 is a sectional view showing a low-EMI cable connector accordingto a further embodiment of the present invention,

FIG. 12 is a sectional view showing a low-EMI cable connector accordingto a still further embodiment of the present invention, and

FIG. 13 is a diagram for explaining the generation of the unrequitedradiation from the transmission cable.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be explained below withreference to the drawings.

FIG. 1 is a sectional view showing a low-EMI circuit board according toan embodiment of the present invention. 1 designates a multilayercircuit board, 2 a ground layer, 2 a loss layers, 3 a power layer, 3 aloss layers, 4 signal line layers, 5 through holes, 6 a to 6 n electrodepatterns, 7 solders, 8 an LSI element, 9 an IC element, 10 matchingtermination resistor elements, 11 a resistor element, 12 a shield plate,13 conductive foils, 14 loss layers, 15 a, 15 b, 16 insulating layers,and 17 a dielectric layer.

In the drawing, this embodiment is configured in such a manner that theshield plate 12 is mounted on the surface of the multilayer circuitboard 1 with the ground layer 2, the power layer 3 and the signal linelayers 4 arranged thereon. In place of the solders 7 in this case,conductive adhesives are sometimes used for the reason of the heatresistance, especially, when the shield plate 12 is mounted.

This multilayer circuit board 1 has the ground layer 2 and the powerlayer 3 arranged in the dielectric layer 17, and a single signal linelayer 4 is arranged between one surface layer and the ground layer 2,two signal line layers 4 are arranged between the ground layer 2 and thepower layer 3, and a single signal line layer 4 is arranged between thepower layer 3 and the other surface layer. Further, a signal line layeris arranged on each surface layer, thereby constituting what is calledan 8-layer board.

An electrode pattern is arranged at a predetermined portion of thesignal line layer 4 formed on each surface layer, and some of them areconnected electrically to the ground layer 2, while others are connectedelectrically to the power layer 3. only the electrode patterns 6 a to 6n are shown here, and in the description that follows, only theelectrode patterns 6 a to 6 n will be referred to as the whole electrodepatterns. The electrode patterns 6, 6 c, 6 e to 6 g, 6 i to 6 k, 6 m, 6n are connected electrically to the ground layer 2 through the throughholes 5. The electrode patterns 6 d, 6 l, on the other hand, areelectrically connected to the power layer 3 through the through holes 5.

It is assumed that a terminal of the resistor element 11 making up thedesired circuit is mounted on the electrode pattern 6 d connected to thepower layer 3 and the electrode pattern 6 c connected to the groundlayer 2, that the lead of the LSI element 8 molded to the electrodepattern 6 d and the electrode pattern 6 e connected to the ground layer2 is mounted by the solders 7, and that the lead of the packaged ICelement 9 is mounted by the solders 7 on the electrode pattern 6 lconnected to the power layer 3 and the electrode pattern 6 k connectedto the ground layer 2. In the process, the IC element 9 may be mountedin the form of a bare chip.

The shield plate 12 is arranged in such a manner as to cover the twowhole surfaces of the multilayer circuit board 1 including the LSIelement 8, the IC element 9 and the resistor element 11 mounted thereon.The shield plate 12, which is made of a thin plate having a completelyflat surface, may have a multiplicity of small holes for securing theradiation of heat from the IC element, the LSI element, etc. In theprocess, the hole diameter (shape) ø is assumed to be not more than λ/60(λ is the radiation wavelength) taking the area of suppressing theunrequired radiation into consideration. The shield plate 12 and theground layer 2 are used to suppress mainly the radiation of differentialmode.

The shield plate 12 is mounted by the solders 7 on the electrodepatterns around the multilayer circuit board 1. FIG. 1 shows theelectrode patterns 6 a, 6 g, 6 h, 6 n as the electrode patterns aroundthe multilayer circuit board 1.

The shield plate 12 is such that the two surfaces of the conductive foil(such as a copper foil) having the loss layer 14 on the surface thereofnearer to the multilayer circuit board 1 are covered with insulatinglayers 15 a, 15 b, and due to the loss layer 14, forms a line with alarge attenuation constant α with the ground layer 2. The insulatinglayer 15 a covering the surface of the conductive foil 13 on the losslayer 14 side is made of a heat-resistant insulating film such as ofpolyimide or polyester. The insulating layer 15 b on the opposite sideis formed of a resist material or the like printed, coated or sprayed inorder to prevent the oxidation or assure the electrical insulation ofthe conductive film 13. Of course, the insulating layer 15 b is notessential, but in the absence thereof, the surface of the conductivelayer 13 is subjected to an anti-oxidation treatment. Also, theinsulating film described above may be used.

The shield plate 12 is electrically connected to the electrode patterns6 a, 6 g, 6 h, 6 n with the conductive foil 13 connected with thesolders 7.

The loss layers 14 may be nickel plated layers or chromium platedlayers, for example, having a large resistivity. As an alternative, thesurface of the conductive foil 13 can be roughened so that thetrans-mission path is lengthened to increase the resistance valueequivalently. In such a case, the adhesion is improved between theconductive layer 13 and the insulating film such as of polyimideconstituting the insulating layer 15 a.

Also, the shield plate 12 is desirably flexible and formed with a die inaccordance with the roughness of the component parts mounted on themultilayer circuit board 1. If formed with a die in this way, themounting on the multilayer circuit board 1 is easy. Also, the portionsmounted on the electrode patterns 6 a, 6 g, 6 h, 6 n are reasonablyarranged proximate to the electrode patterns 6 a, 6 g, 6 h, 6 n,respectively, thereby simplifying the soldering work.

According to this embodiment, the radiation of differential modegenerated from the LSI element 8 or the IC e element 9 is containedbetween the shield pate 12 and the ground layer 2. The mere provision ofthe shield plate 12 independent of the ground layer 2, however, wouldgenerate the radiation of differential mode by the high harmonics of thefast clock from the fast operating parts or the signal processed basedon the fast clock, whereby energy is accumulated between the shieldplate 12 and the ground layer 2, and the resulting potential differencebetween the shield plate 12 and the ground layer 2 causes a resonance.Thus, the reflection occurring in the peripheral portion thereofgenerates a standing wave, and an electric field is generated in theperipheral portion of the shield plate 2 and the ground layer 2 therebyto generate the unrequired radiation. In other words, the shield plate12 suppresses the radiation of differential mode, with the result that anew radiation of common mode would be caused.

In order to prevent this phenomenon, according to this embodiment, theshield layer 12 and the ground layer make up substantially parallellines. These lines are electrically connected at a termination, therebymaking it difficult for the standing wave to occur in the shield plate12. In this way, the unrequired radiation is suppressed from theperipheral portion of the shield plate 12. Methods of connection include

(1) connecting through a resistor element having a resistance value nearto the matching termination resistance value, and

(2) attaching the function as a shield by direct connection. FIG. 1shows the case in which the shield plate 12 is connected to the groundlayer 2 by these two methods. In spite of this, one of the two methodscan be employed.

First, the method (1) will be described.

The electrode patterns 6 a, 6 h arranged on the periphery of the surfacelayer of the multilayer circuit board 1 are connected with the shieldplate 12 by the solders 7. A terminal of a chip resistor 10 of apredetermined resistance value is mounted by the solders 7 on theelectrode pattern 6 a and the electrode pattern 6 b around themultilayer circuit board 1 which is connected to ground layer 2 via thethrough hole 5, in such a manner as to produce a termination resistancevalue proximate to the matching termination resistance value. Also, aterminal of the chip resistor 10 of a predetermined resistance value ismounted by the solders 7 on the electrode pattern 6 h and the electrodepattern 6 i around the multilayer circuit board 1 connected to theground layer 2 via the through hole 5. Specifically, the peripheralportion of the shield plate 12 is connected to the ground layer 2through the chip resistor 10. Actually in this case, however, in view ofthe requirement of the termination in the fashion of a distributedconstant, a plurality of chip resistors 10 are connected in parallel forsubstantially equidistant termination at a pitch determined by thestructure of the multilayer circuit board 1 (25 to 50 mm, for example,in the case of an 8-layer board of about A4 size) in the peripheralportion of the multilayer circuit board 1. In such a case, theinductance component depending on the structure of the through holes 5and the electrode patterns 6 a, 6 b, 6 h, 6 i is set to a sufficientlysmall level as compared with the termination resistance value.

As a result, the unrequired radiation of common mode is suppressed whichis newly generated from the peripheral portion of the shield plate 12which has shielded the radiation of differential mode.

Next, the method (2) will be explained.

The peripheral portion of the shield plate 12 is connected by thesolders 7 to the electrode patterns 6 g, 6 n, etc. electricallyconnected to the ground layer 2 via the through hole 5. Specifically,the shield plate 12 and the ground layer 2 are directly connectedelectrically only via the through hole 5, so that the shield plate 12assumes the same potential as the ground layer 2.

With this configuration, the radiation of differential mode is generatedfrom the LSI element 8 and the IC element 9 adapted for fast operationand the signal line layers (high-speed signal line layers) 4 fortransmitting the high frequency signal and the clock (such componentparts mounted and the high-speed signal line layers 4 are hereinaftercalled the fast operating parts). However, the fast operating partsproviding a radiation source are shielded in such a manner as to beenclosed by the ground layer 2 and the shield plate 12 via the throughholes 5. Thus the radiation is contained between the ground layer 2 andthe shield plate 12, and at the same time, the shield layer 12 and theground layer 2 are set to the same potential by being electricallyconnected to each other via the through holes 5. Therefore, a standingwave, even if generated in the shield plate 12 and the ground layer 2,fails to emerge so that the unrequired radiation can be effectivelysuppressed also from the peripheral portion of the shield plate 12.

As described above, the short-circuiting the ground layer 2 and theshield plate 12 via the through holes 5 also transfers the resonancefrequency of the loop involved to a frequency region higher than theregion for suppressing the unrequired radiation. As a result, theresonance in this frequency region is suppressed thereby to reduce theunrequired radiation in this frequency region. This frequency transfercan be not less than 1 GHz, whereby the frequency of the unrequiredradiation can be transferred to a frequency band free of regulation.

Also, the generation of the radiation of differential mode describedabove leads to the generation of a high frequency current due to thestanding wave in the loop configured of the ground layer 2, the throughholes 5 and the shield plate 12. This high frequency current, however,flows along the inner surface of the conductive foils 13 (along thesurface on the multilayer circuit board 1 side) of the shield plate 12in opposed relation to the ground layer 2. The provision of the losslayer 14 on the particular surface attenuates by converting the highfrequency current into Joule heat. As a result, the generation of thestanding wave is suppressed, whereby the unrequired radiation generatedfrom the peripheral portion of the shield plate 12 is also reduced.

In the above-mentioned manner, the shield plate 12 and the ground layer2 are connected to each other directly or through a matching terminationresistor. Thus, the radiation of differential mode can be effectivelysuppressed while at the same time suppressing the new generation of theradiation of common mode.

The LSI element and the drive IC element generates an especially greatamount of the radiation of differential mode. These circuit elements,therefore, are shielded with double connection areas using the shieldplate 12. Specifically, assume that the LSI element 8 and the IC element9 are involved. The electrode patterns such as the electrode patterns 6f, 6 j, 6 m electrically connected to the ground layer 2 at a pitchcorresponding to the operating frequency are arranged around the LSIelement 2 and the IC element 9, and are electrically connected to theshield plate 12 by soldering. In other words, the peripheral portions ofthe LSI element 8 and the IC element 9 on the shield plate 12 areconnected at multiple points with the ground layer 2 and shieldedpartially by the shield plate 12. As a result, the unrequired radiationof differential mode is contained by the current loop including theshielded portion of the shield plate 12 and the ground layer 2, andsuppressed by the loss layers 14.

In this way, the radiation of differential mode generated by the LSIelement 8 and the IC element 9 is shielded at the periphery thereof bythe shield plate 12 and the ground layer 2. At the same time, theradiation of differential mode is also shielded by the periphery of themultilayer circuit board 1 for double shielding. The radiation isabsorbed by each of the shields. Further, the strong radiation of commonmode generated from these parts is effectively suppressed.

Though not shown, the signal line layers 4 inserted are also connectedto the power layer 3 and the ground layer 2 directly or through themounted component parts. Especially upon activation of a fast operatingpart, a high frequency current flows in the ground layer 2 and the powerlayer 3. This high frequency current flows also in the peripheralportion of the ground layer 2 and the power layer 3, and the resonancebetween them causes the radiation of common mode.

In order to prevent this, according to this embodiment, loss layers 2 a,3 a are arranged along the peripheral portion of the multilayer circuitboard 1 in the ground layer 2 and the power layer 3 thereby to attenuatethe high frequency current due to the resonance. The loss layers 2 a, 3a are arranged in the range outside of the junction point between theground layer 2 or the power layer 3 and the signal line layers via thethrough hole 5 not to interfere with the operation of the parts and thecircuits mounted on the multilayer circuit board 1. The loss layers 2 a,3 a can also be similar to the loss layer 14 of the shield layer 12.

FIG. 2(a) shows the loss layer 2 a of the ground layer 2. The groundlayer 2 is arranged substantially over the entire section of themultilayer circuit board 1, and therefore the loss layer 2 a is arrangedover the entire peripheral portion thereof. FIG. 2(b) shows the losslayer 3 a of the power layer 3. In this case, four different powerlayers 3A, 3B, 3C, 3D with different source voltages are assumed toexist. The loss layer 3 a is arranged in the portion of the power layer3 corresponding to the peripheral portion of the multilayer circuitboard 1. In the case of FIG. 2(b), therefore, the loss layer 3 a isarranged in a part of the peripheral portion of the power layers 3A, 3B,3C, but not in the power layer 3D arranged at the central part of themultilayer circuit board 1.

As described above, according to this embodiment, the radiation ofdifferential mode generated from the mounted component parts and thesignal line layer can be very effectively suppressed.

In this embodiment, the shield plate 12 is fixed on the multilayercircuit board 1 by the solders 7. By removing the solders 7, therefore,the shield plate 12 can be easily removed and the mounted parts can beeasily replaced. Consequently, any one of the mounted parts, if it runsout of order, can be easily replaced to reuse the circuit board. Fromthe viewpoint of heat resistance and environmental protection, aconductive adhesive (a thermo-setting resin containing Ag (Cu, Au)powder, for example) is sometimes used instead of the solders 7.

The shield plate 12 is arranged on the two sides of the multilayercircuit board 1 in this embodiment. In the case where a fast operatingpart is mounted or the signal line layers are arranged only on one side,however, it is obviously sufficient to arrange the shield plate 12 onlyon the particular side.

Further, in the multilayer circuit board 1, the more the high-speedsignal lines providing a radiation source are arranged in the peripheralportion of the multilayer circuit board 1, the more radiation ofdifferential mode is generated from the particular peripheral portion.The high-speed signal lines, therefore, are preferably arranged on thecentral portion in the multi-layer circuit board 1.

FIG. 3 is a sectional view showing a low-EMI circuit board according toanother embodiment of the present invention. 6 p, 6 q, 6 r designateelectrode patterns, 18 a dielectric layer, 19 a ground layer and 20 aresistor element. The same parts as the corresponding parts in FIG. 1are designated by the same reference numerals, respectively, and willnot be explained again. The reference numerals are also partly omitted.

In this embodiment, not only the radiation of differential mode issuppressed, but also the radiation of common mode can also be suppressedanew.

In FIG. 3, another ground layer 19 is arranged between the power layer 3and the signal line layer 4 on the side of the power layer 3 far fromthe ground layer 2, and a dielectric layer 18 having a dielectricconstant εr different from the dielectric layer 17 is arranged betweenthe ground layer 19 and the power layer 3. This dielectric layer 18forms a capacitor C with the power layer 3 and the ground layer 19 aselectrodes, and is configured to exhibit a sufficiently small impedanceagainst the high harmonics such as a fast clock.

A resistor element 20 is connected between the electrode patterns 6 pand 6 q on one of the surface layers on the peripheral portion side ofthe multilayer circuit board 1, and another resistor 20 is connectedbetween the electrode patterns 6 m and 6 r on the other surface layer.The electrode patterns 6 p, 6 r are connected to one ground layer 2 viathe through holes 5, while the electrode patterns 6 q, 6 m are connectedto the other ground layer 19 via the through holes 5.

Therefore, the power layer 3 and the ground layer 19 are connected toeach other through the capacitor C including the dielectric layer 18,and the ground layer 19 is connected to the ground layer 2 through theresistor element 20.

With this configuration, even when the potential of the power layer 3tends to fluctuate due to the high harmonics such as the fast clock, theharmonics current is supplied to the resistor element 20 through thecapacitor C including the dielectric layer 18 and further flows to theground layer 2. In the process, the harmonics current is attenuated bybeing converted into Joule heat in the resistor element 20. As a result,the potential fluctuation due to the high harmonics generated betweenthe ground layer 2 and the power layer 3, especially, the resonancecurrent is suppressed, thereby suppressing the radiation of common mode.

A capacitor C′ constituting an interlayer stray capacity due to thedielectric layer 17 is interposed between the power layer 3 and theground layer 2. This capacitor C′ has so high a Q that the mounted partssuch as the LSI element, IC element and the chip capacitor come to holdan impedance component of inductive characteristic in high harmonicsregion. The parallel resonance of these components generates theradiation of common mode.

According to this embodiment, however, the series circuit including thecapacitor C due to the dielectric layer 18 and the resistor element 20is connected in parallel to the capacitor C′. Therefore, when theimpedance of the capacitor C is set sufficiently small as compared withthe resistance value of the resistor element 20, the capacitor C isignored and the resistor element 20 is connected in parallel to thecapacitor C′. The inductance component due to the electrode patterns 6p, 6 r and the through holes 5 generated at the time of packaging is setin such a manner that the impedance can be sufficiently ignored ascompared with the resistor element 20. By setting the resistance valueof the resistor element 20 to an appropriately small value, the Q of thecapacitor C′ can be reduced. As a result, the energy consumption in theresistor 20 increases, so that the potential fluctuation in the powerlayer 3 is effectively absorbed by the resistor element 20 thereby toreduce the radiation of common mode.

The capacitor C is for cutting DC, i.e. for cutting the DC voltage fromthe power layer 3 to the ground layer 19.

Also, according to this embodiment, the shield plates at upper and lowersides of the drawing are connected to the ground layer 2 and the groundlayer 19 by the through holes 5 and the electrode patterns 6 n, 6 q,etc. of the peripheral portion of the multilayer circuit board 1. As aresult, each of the two surfaces of the board 1 has a structure to beshielded by the shield 12, the ground layer 2, the ground layer 19 andthe through holes 5 at multiple points. Thus, the radiation ofdifferential mode due to the mounted component parts and the signal linelayer 4 arranged in the shield structure is suppressed.

FIG. 4(a) is a longitudinal sectional view showing a low-EMI cableconnector according to a first embodiment of the invention, and FIG.4(b) is a cross sectional view taken in line X—X in FIG. 4(a). 21designates a cylindrical portion of the base, 21 a a female screwportion, 21 b a soldered portion, 22 a bottomed cylindrical portion, 22a a short-circuiting termination portion, 22 b a dielectric portion, 23a bottomed cylindrical portion, 23 a a short-circuiting terminationportion and 23 b a dielectric portion.

In this drawing, the bottomed cylindrical portion 22 of a conductorhaving a length l₁ is arranged on the outer peripheral surface of thebase cylindrical portion 21 made of a conductor, and the bottomedcylindrical portion 23 made of a conductor having a length l₂ isarranged on the outer peripheral surface of the bottomed cylindricalportion 22. These two bottomed cylindrical portions 22, 23 are laid oneon the other concentrically. The bottomed cylindrical portion 22 has thedepth thereof closed by the short-circuiting termination portion 22 a,thereby forming a line terminated by short-circuiting. Similarly, thebottomed cylindrical portion 23 has the depth thereof closed by theshort-circuiting portion 23 a thereby to form a line terminated byshort-circuiting. Also, a dielectric material of a dielectric constantεr1 is filled to form a dielectric portion 22 b in the bottomedcylindrical portion 22. In similar fashion, a dielectric material of adielectric constant εr2 is filled to form a dielectric portion 23 b inthe bottomed cylindrical portion 23.

A coaxial cable is fitted on the inner diameter of the base cylindricalportion 21, and for this purpose, a female screw portion 21 a is formedto be screwed with a male screw portion formed on the outer peripheralsurface of the coaxial cable not shown. Also, a solder portion 21 b isformed with the Au plating or the like applied at the end of the outerperipheral surface of the base cylindrical portion 21 far from theportion of the same outer peripheral surface where the bottomedcylindrical portion 22 is arranged.

The short-circuiting termination line of length l₁ including thebottomed cylindrical portion 22 resonates at λ01/(2n−1), where n is 1,2, 3, . . . , for the wave-length λ01 satisfying the relation l₁=λ01/4,and the impedance Z01 as viewed from the open side (shown by arrow) ofthe bottomed cylindrical portion 22 becomes substantially infinitelylarge at the resonance point. In similar fashion, the short-circuitingtermination line of length l₂ including the bottomed cylindrical portion23 resonates at λ02/(2n−1) for the wavelength λ02 satisfying therelation l₂=λ02/4, and the impedance Z02 as viewed from the open side(shown by arrow) of the bottomed cylindrical portion 23 becomes almostinfinitely large at the resonance point.

Assume that the angular frequency (resonance angular frequency) for thewavelength λ01 is ω0 (=2πf0).

The impedance Z01 of the short-circuiting termination line including thebottomed cylindrical portion 22 changes as shown by solid line in FIG. 5according to the angular frequency ω, and becomes almost infinitelylarge by resonating at the angular frequencies ω0, 3ω0, 5ω0, 7ω0, 9ω0,.. . Also, assuming that λ02=λ01/2, the resonance angular frequency isdouble the above-mentioned resonance frequency, the impedance Z02 of theshort-circuiting termination line including the bottomed cylindricalportion changes as shown by dashed line in FIG. 5 according to theangular frequency ω, and becomes almost infinitely large by resonatingat the angular frequencies 2ω0, 6ω0, 10ω0, . . . . These impedance Z01,Z02 converge to the matching termination resistance values R01, R02,respectively, of the respective lines.

Next, explanation will be given of the case in which this low-EMI cableconnector is used for the transmission line with the coaxial cable shownin FIG. 13.

FIG. 6 is a view showing the state in which the low-EMI cable connectoris arranged at the end of the coaxial cable 102 nearer to thetransmitting terminal unit 100 in FIG. 13. The same component parts asthe corresponding ones in the preceding drawing are designated by thesame reference numerals, respectively.

In this drawing, the low-EMI cable connector is mounted at the end ofthe coaxial cable 102 in such a position that the open side of thebottomed cylindrical portions 22, 23 is directed toward the transmittingterminal unit 100. For this mounting, as explained with reference toFIG. 4, the male screw portion (not shown) arranged at the end of thecoaxial cable 102 is screwed into the female screw portion 21 a arrangedin the base cylindrical portion 21 of the low-EMI cable connector. Also,the solder portion 21 b of the base cylindrical portion 21 is solderedto the case or the like of the transmitting terminal unit 100. In FIG.6, however, the base cylindrical portion 21 is not shown.

As explained with reference to FIG. 13, the current i2 a that has flowedtoward the transmitting terminal unit 100 along the inner surface of theexternal conductor 102 b of the coaxial cable 102 partly leaks out atthe junction point A between the external conductor 102 b of the coaxialcable 102 and the inward line 100 b of the transmitting terminal unitand tends to flow along the outer surface of the external conductor 102b.

Let i3 a be the leakage current of a wavelength λ0 (=c/f0) correspondingto the fundamental frequency (clock frequency) f0 of the transmissionsignal, times 1/(2n−1) (n: 1, 2, 3, . . . ), i4 a be the leakage currentof a wave-length λ0 times 1/{2×(2n−1)}, and i5 a be the leakage currentof other wavelengths. Also, assume that the coaxial cable 102 resonatesat the fundamental frequency f0 (hence, also at frequencies (2n−1)·f0equal to odd multiples thereof).

These currents i3 a, i4 a, i5 a flow into the short-circuitingtermination line including the bottomed cylindrical portion 22 from thejunction point A.

In the short-circuiting termination line including the bottomedcylindrical portion 22 of length l₁, as described above, a frequency(2n−1) times the frequency f01 (=c/λ01, where c is the velocity oflight) for the wavelength λ01 satisfying the relation l₁=λ01/4 is aresonance frequency at which the impedance Z01 becomes substantiallyinfinitely large. When a current of wavelength λ flows in the dielectricmaterial of dielectric constant εr, on the other hand, the wavelength ofthe current in this dielectric material is shortened to λ/{square rootover ( )}εr.

In view of this, consider the current i3 a among the currents i3 a, i4a, i5 a flowing into the short-circuiting termination line including thebottomed cylindrical portion 22. The following relation is obtained forthe wavelength λ3 a of this current i3 a in the short-circuitingtermination line. $\begin{matrix}{{\lambda \quad 0^{\prime}} = \frac{\lambda 0}{\sqrt{ɛ_{r1}}}} & \left( {{Expression}\quad 3} \right)\end{matrix}$

The length l₁ of the short-circuiting termination line including thebottomed cylindrical portion 22 is set in such a manner that theresonance occurs at this wavelength λ0′, i.e. in such a manner thatλ0′=λ01. In this way, the impedance Z01 of the short-circuitingtermination line including the bottomed cylindrical portion 22 becomessubstantially infinitely large for the current i3 a of the wavelengthλ0/ (2n−1). This current i3 a can thus be suppressed almost completely.

Under this condition, the length l₁ of the short-circuiting terminationline including the bottomed cylindrical portion 22 is given as$\begin{matrix}{1_{t} = {\frac{{\lambda 0}^{\prime}}{4} = \frac{\lambda 0}{4 \cdot \sqrt{ɛ_{r1}}}}} & \left( {{Expression}\quad 4} \right)\end{matrix}$

In this way, the leakage current i3 a can be reduced substantially tozero in the short-circuiting termination line including the bottomedcylindrical portion 22. The other leakage currents i4 a, i5 a, however,though attenuated to some degree, pass through the short-circuitingtermination line including the bottomed cylindrical portion 22 with thesame wavelength, for lack of resonance in the short-circuitingtermination line including the bottomed cylindrical portion 22, and flowinto the short-circuiting termination line including the bottomedcylindrical portion 23.

The leakage current i4 a is suppressed in the short-circuitingtermination line including the bottomed cylindrical portion 23. For thispurpose, the length 12 of the short-circuiting termination lineincluding the bottomed cylindrical portion 23 is set in such a mannerthat the short-circuiting termination line including the bottomedcylindrical portion 23 resonates at the frequency of the leakage currenti4 a.

The wavelength λ1 of this current i4 a is given as λ1=λ0/2 as describedabove. Assuming that the dielectric constant εr2 of the dielectricmaterial of the dielectric portion 23 b (FIG. 4) in the bottomedcylindrical portion 23 is equal to the dielectric constant εrl of thedielectric material of the dielectric portion 22 b (FIG. 4) in thebottomed cylindrical portion 22, i.e. εr2=εr1, then, from Expression 4above,

l ₂ =l ₁/2

This shows that the impedance Z02 of the short-circuiting terminationline including the bottomed cylindrical portion 23 exhibits acharacteristic indicated by dashed line in FIG. 5, and the leakagecurrent i4 a of angular frequencies 2ω0, 6ω0, 10ω0, . . . can besuppressed substantially to zero.

As described above, the coaxial cable 102 envelops a resonance at thefundamental frequency f0 and a frequency an odd multiple thereof, andthe frequencies even multiples of the fundamental frequency f0 pose noproblem. In this case, therefore, the bottomed cylindrical portion 23 isnot necessarily required. Nevertheless, the reason why the bottomedcylindrical portion 23 is provided is described later.

FIG. 7 shows an equivalent circuit of the configuration described above.24 designates a short-circuiting termination line including the bottomedcylindrical portion 22 of the low-EMI cable connector on thetransmitting terminal unit 100 side, 24′ a short-circuiting terminationline including the bottomed cylindrical portion 22 of the low-EMI cableconnector on the receiving terminal unit 101 side, 25 a short-circuitingtermination line including the bottomed cylindrical portion 23 of thelow-EMI cable connector on the transmitting terminal unit 100 side, and25′ ma short-circuiting termination line including the bottomedcylindrical portion 23 of the low-EMI cable connector on the receivingterminal unit 101 side. Za, Zb designate the impedance of the externalconductor 102 b, etc. of the coaxial cable 102.

In this way, on the transmitting terminal unit 100 side, theshort-circuiting termination lines 24, 25 including the bottomedcylindrical portions 22, 23 can suppress substantially to zero thecurrents of the frequencies f0, 2f0, 3f0, 5f0, 6f0, 7f0, 9f0, 10f0, . .. reflected at the junction point A by the equivalent impedancegenerated between the junction point A and the grounding point A′ forthe clock frequency f0 (=ω0/2π) of the transmitted signal. The leakagecurrent i5 a having other frequencies can be applied directly along theouter surface of the external conductor 102 b of the coaxial cable 102from the short-circuiting termination lines including the bottomedcylindrical sections 22, 23 without any problem, as long as theamplitude thereof is sufficiently small or the coaxial cable 102 has nofrequency causing the resonance. In the case where the coaxial cable 102oscillates and generates the unrequired radiation, however, ashort-circuiting termination line including a bottomed cylindricalportion for suppressing the current i5 a is arranged further outside ofthe bottomed cylindrical portion 23.

This is also the case with the receiving terminal unit 101 side, wherethe current of the above-mentioned frequencies reflected at the junctionpoint B by the impedance generated between the junction point B and thegrounding point B′ is sufficiently suppressed by the short-circuitingtermination lines 24′, 25′ including the bottomed cylindrical sections22, 23 of the low-EMI cable connector.

As clear from Expression 4 above, a dielectric material having largedielectric constants εr1, εr2 is used for the dielectric portions 22 b,23 b of the bottomed cylindrical portions 22, 23, thereby making itpossible to shorten the lengths l₁, l₂ of the bottomed cylindricalportions 22, 23. The dielectric material of strontium titanate or bariumtitanate has a dielectric constant of 300 to 1000.

In view of this, assume that a dielectric material having a dielectricconstant εr1 of 900 is used for the dielectric portion 22 b of thebottomed cylindrical portion 22 and that the clock frequency f0 is 200MHz,

λ0=c/f0=3×108/(200×106)=150 cm

Therefore, from Expression 4,

l ₁=150/(4×{square root over ( )}900)=1.25 cm

Also, if the same dielectric material is used for the dielectric portion23 b of the bottomed cylindrical portion 23, the length l₂ of thebottomed cylindrical portion 23 is l₂=l₁/2=0.625 cm.

As described above, an impedance extremely high as compared with 100 Ωfor the conventional ferrite core can be obtained by a cable connectormade of a short-circuiting termination line as short as less than 2 cmfor the very high clock frequency of 200 MHz. Thus, the unrequiredradiation from the coaxial cable 102 can be suppressed almostcompletely.

Also, the thickness of the bottomed cylindrical portions 22, 23 is suchthat the short-circuiting termination lines including them have aninfinitely large impedance at a predetermined frequency and requires nomatching termination resistor of a specific value. The bottomcylindrical sections 22, 23, therefore, can be thinned arbitrarily. Forthis reason, in this embodiment, which is compact, light in weight andnot bulky, the occurrence of the unrequired radiation from the coaxialcable 102 can be suppressed with a sufficiently high effect.

In FIG. 4(a), the bottomed cylindrical portions 22, 23 can be integrallyformed on the base cylindrical portion 21. Instead, the bottomedcylindrical portions 22, 23 can be formed as separate cylindricalportions having dielectric portions 22 b, 223 b, respectively, with afemale screw on the inner surface of the bottomed cylindrical portions22, 23 and a male screw on the outer peripheral surface of the basecylindrical portion 21 and the bottomed cylindrical portion 22. Byscrewing these screws, the bottomed cylindrical portion 22 can bemounted on the base cylindrical portion 21, and further, the bottomedcylindrical portion 23 can be mounted on the bottomed cylindricalportion 22. In such a case, the position of the bottomed cylindricalportion 22 relative to the base cylindrical portion 21 and the positionof the bottomed cylindrical portion 23 relative to the bottomedcylindrical portion 22 can be appropriately adjusted.

Once the low-EMI cable connector is rendered mountable on the coaxialcable 102 by the screws in this way, low-EMI cable connectors forsuppressing different frequencies can be provided, so that the desiredlow-EMI cable connector can be selectively used in accordance with thelength of the coaxial cable (hence, the resonance frequency).

In the foregoing description, the coaxial cable 102 is mounted by beingscrewed to the base cylindrical portion 21 by the female screw 21 a orthe like (whereby the cable becomes replaceable). As an alternative,however, the base cylindrical portion 21 can of course be integrallyfixed with the coaxial cable 102.

Further, the dielectric portions 22 b, 23 b of the bottomed cylindricalportions 22, 23 can of course be made using dielectric materials ofdifferent constants instead of dielectric materials of the samedielectric constant. In such a case, the length of the bottomedcylindrical portions 22, 23 can be rendered substantially equal to eachother by use of a dielectric material of a small dielectric constant forthe dielectric portion 23 b of the bottomed cylindrical portion 23 ascompared with the dielectric constant of the dielectric material of thedielectric portion 22 b of the bottomed cylindrical portion 22.

FIG. 8 is a diagram showing the state in which the low-EMI cableconnector shown in FIG. 4 is arranged at the end of the coaxial cable102 nearer to the receiving terminal unit 101 (i.e. the low-EMI circuitboard described with reference to FIGS. 1 to 3) in FIG. 13. The samecomponent parts as the corresponding parts in the drawings aredesignated by the same reference numerals, respectively.

In the drawing, the coaxial cable 102 with the low-EMI cable connectormounted at an end thereof in the above-mentioned manner is connected tothe input/output terminal of the low-EMI circuit board. In the process,the internal conductor 102 a of the coaxial cable 102 is connectedelectrically by the solders 7 to the electrode pattern 6 arranged on thesignal line layer 4 of the low-EMI circuit board. Also, the externalconductor 102 b of the coaxial cable 102, with the forward end thereofexpanded, is connected electrically by the solders 7 to the ground layer2 of the coaxial cable 102. In this case, the forward end of theexternal conductor 102 b is not soldered in it entirety, but generallyat multiple points, say, four points and is very difficult tohermetically seal.

With this configuration, the signal current i1 b that has flowed inthrough the internal conductor 102 a of the coaxial cable 102 flows intothe signal line layer 4 of the low-EMI circuit board through theelectrode pattern 6. The inward current i2 b flowing along the innersurface of the ground layer 2, on the other hand, flows along the innersurface of the external conductor 102 b of the coaxial cable 102. Sincethe external conductor 102 b is soldered at, say, four points but notcompletely sealed, however, part of the inward current i2 b leaks to theouter surface of the external conductor 102 b from the gap not solderedof the external conductor 102 b of the coaxial cable 102, and tends toflow along the outer surface of the external conductor 102 b as acurrent i3 b.

In contrast, a low-EMI cable connector is mounted at the joint betweenthe coaxial cable 102 and the low-EMI circuit board, so that the leakagecurrent i3 b flows into his low-EMI cable connector. In the process, asin the case of FIG. 6, the leakage current i3 b of a frequency causingthe unrequired radiation in the coaxial cable 102 can be suppressedsubstantially to zero by setting the length of the bottomed cylindricalportions 22, 23 of the low-EMI cable connector appropriately and using adielectric material of a proper dielectric constant for the bottomedcylindrical portions 22, 23.

Of course, the current flowing into the coaxial cable 102 from thelow-EMI circuit board is not limited to the current flowing along theinner surface of the ground layer 2 but also includes the currentflowing along the outer surface of the ground layer 2 arranged on theboard surface and the current flowing along the surface of theconductive foil 13 of the shield plate 12. These currents partly leakout to the outer surface of the external conductor 102 b of the coaxialcable 102 due to the presence of the gap caused by the multi-pointconnection between the forward end of the external conductor 102 b ofthe coaxial cable 102 and the ground layer 2. The low-EMI cableconnector functions effectively even against this leakage current asdescribe above.

In the embodiment shown in FIGS. 4 to 7, the low-EMI cable connector isconfigured in two stages having two bottomed cylindrical portions 22,23. The unrequired radiation is generated by resonance of the coaxialcable 102 from the fundamental wave of the fundamental wave frequency f0and odd-multiple high harmonics (2n−1)·f0 thereof. This can be preventedby increasing the impedance Z01 of the low-EMI cable connector to aninfinitely large value for the fundamental frequency ω0 and oddmultiples 3ω0, 5ω0, . . . , thereof, as shown by solid line in FIG. 5.From this point of view, the low-EMI cable connector having one-stageconfiguration having only the bottomed cylindrical portion 22 issufficient in FIGS. 4 to 8.

In some cases, however, a connection equivalent to the connection to theground line is obtained by providing a grounding line at an appropriatepoint of the external conductor 102 b of the coaxial cable 102 or bybringing the external conductor 102 b into contact with the ground. Insuch a case, not only the coaxial cable 102 itself but also the lineincluding the part of the coaxial cable up to the connection pointthereof with the ground line and the ground line resonates and generatesthe unrequired radiation at a frequency 2ω0 twice as high as thefundamental frequency ω0. Then, as shown in FIGS. 4 to 8, the provisionof a short-circuiting termination line including a bottomed cylindricalportion 23 with the impedance Z02 becoming infinitely large at thefrequency 2ω0 on the outside of the bottomed cylindrical portion 22 cansuppress the generation of the unrequired radiation due to the resonanceof the frequency 2ω0.

Also, in the case where a plurality of coaxial cables are arranged inparallel, a single low-EMI cable connector can enclose them collectivelyand thus can simultaneously suppress the currents causing the unrequiredradiation in the coaxial cables. In such a case, due to the differenceof length, some coaxial cables may develop the resonance at thefundamental frequency ω0 and odd multiples thereof, and others at evenmultiples of the fundamental frequency ω0. In such a case, too, as shownin FIGS. 4 to 8, the low-EMI cable connector can be configured in twostages to prevent the unrequired radiation as a whole.

A similar effect is obtained for the cable in which a multiplicity ofsignal lines and ground lines are collectively shielded, for which asimilar low-EMI cable connector can be used.

FIG. 9 is a longitudinal sectional view showing a low-EMI cableconnector according to a second embodiment of the preset invention. 22′designates a cylindrical portion, and 22 c a matching terminationresistor. The same component parts as the corresponding parts in thepreceding drawings are designated by the same reference numerals,respectively.

In the drawing, this embodiment comprises the matching terminationresistor 22 c instead of arranging a short-circuiting termination in thedepth of the unbottomed cylindrical portion 22′. The remainingconfiguration is similar to that of the embodiment described withreference to FIGS. 4 to 8.

According to this embodiment, the length of the cylindrical portion 22 cis not important. Since the matching termination resistor 22 c isprovided in the line having the cylindrical portion 22′ with thedielectric portion 22 b, however, the leakage current flowing in thisline is not reflected but flows to the matching termination resistor 22,at which it is converted into thermal energy, thereby suppressing theleakage current.

In similar fashion, in the embodiment of FIGS. 4 to 8, a slight gap inthe short-circuiting termination portions 22 a, 23 a of the bottomedcylindrical portions 22, 23 may cause a current to leak from the gap andflow along the outer surface of the external conductor 102 b of thecoaxial cable 102. In view of this, a matching termination resistorcompatible with the line including the bottomed cylindrical portions 22,23 can be arranged in the short-circuiting termination portions 22 a, 23a so that the current leaking from the gap can be thermally consumed bythe matching termination resistor.

FIG. 10 is a longitudinal sectional view showing a low-EMI cableconnector according to a third embodiment of the invention. 22 a′designates a short-circuiting plate, and the same component parts as thecorresponding parts in the preceding drawings are designated by the samereference numerals, respectively.

In the drawing, according to this embodiment, a lid-shapedshort-circuiting plate 22 a′ of the cylindrical portion 22′ is removableby screw or the like. In the case where the short-circuiting plate 22 a′is mounted on the cylindrical section 22′, like in the first embodimentshown in FIG. 4, the resonance occurs at the frequency f01 for thewavelength λ01 satisfying the relation l₁=λ01/4 and frequenciescorresponding to odd multiples thereof, and the impedance Z01 becomesinfinitely large at these frequencies.

In the case where the short-circuiting plate 22 a′ is removed, on theother hand, an open end results, in which case the short-circuitingtermination is constituted at the position one half the length l₁ of thecylindrical portion 22′. Thus, the impedance Z01′ becomes infinitelylarge at a frequency satisfying the relation

l ₁/2=λ01′/4

This wavelength λ01′ is one half of the wavelength λ01 described above.When the short-circuiting plate 22 a′ is removed, therefore, a frequencytwice as high as the resonance frequency with the short-circuiting plate22′ mounted prevails as a resonance frequency.

This removability of the short-circuiting plate 22 a′ makes it possibleto use cables of two types of different clock frequencies. It ispossible, therefore, to use the same low-EMI cable connector for both acable with the clock frequency of 100 MHz and a cable with the clockfrequency of 200 MHz.

Also this third embodiment, as shown in FIG. 4, can have a double-stagestructure each with a removable short-circuiting plate.

FIG. 11 is a longitudinal sectional view showing low-EMI cable connectoraccording to a fourth embodiment of the present invention. 23′designates a bottomed cylindrical portion, 23 b′ a dielectric portion,and 26 center axis. The same component parts as the corresponding partsin the preceding drawings are designated by the same reference numerals,respectively.

In the foregoing embodiments, a plurality of bottomed cylindricalportions, if any, are arranged coaxially on the base cylindrical portion21. According to the fourth embodiment, in contrast, as shown in FIG.11, the bottomed cylindrical portions are arranged on the basecylindrical portion 21 along the center axis 26 thereof.

The bottomed cylindrical portion 22 resonates at the fundamentalfrequency f0 thereof and frequencies corresponding to odd multiplesthereof. At the resonance frequencies, the impedance Z01 becomessubstantially infinitely large. The bottomed cylindrical portion 23′, onthe other hand, resonates at a frequency twice as high as the resonancefrequency of the bottomed cylindrical portion 22, at which the impedanceZ02 becomes substantially infinitely large.

With this configuration, in addition to a similar effect to that of thefirst embodiment shown in FIGS. 4 to 8, the following effect isobtained. Specifically, even when the bottomed cylindrical portion 22 iselectrically shorted by the short-circuiting termination portion 22 a,the short-circuiting termination portion 22 a may structurally have agap.

Now, let the resonance frequency of the bottomed cylindrical portion 22be fA (=(2n−1)·f0). The bottomed cylindrical portion 22 with thedielectric portion 22 b built therein constitutes a short-circuitingtermination line for the current of frequency fA. The current iB of afrequency different from the frequency fA of the current equal to theresonance frequency fB (=2fA) of the bottomed cylindrical portion 23′leaks out from the structural gap of the short-circuiting terminationportion 22 a of the bottomed cylindrical portion 22. With the low-EMIcable connector shown in FIG. 4, such a leakage current iB cannot besuppressed.

In the embodiment shown in FIG. 11, such a leakage current iB flows intothe next bottomed cylindrical portion 23′, where the current offrequency fB can be substantially suppressed to zero.

FIG. 12 is a longitudinal sectional view showing a low-EMI cableconnector according to a fifth embodiment of the invention. 22′, 23″designate bottomed cylindrical portions, 22 b′, 23 b″ dielectricportions, and 26 a center axis. The same component parts as thecorresponding parts in the preceding drawings are designated by the samereference numerals, respectively.

In this fifth embodiment, the cylindrical portions 22′, 23″ lacking theshort-circuiting termination portion are arranged on the basecylindrical portion 21 along the center axis 26 thereof.

As explained with reference to FIG. 10, the transmission lines includingthe cylindrical portions 22′, 23′ with an open end, which are assumed tohave the lengths of l₁, l₂, respectively, resonate at a frequency equalto odd multiples of the frequency fA′ satisfying the relation l₂/2=λ/4and at a frequency equal to odd multiples of the frequency fB′satisfying the relation l₂/2=λ′/4, respectively, at which the impedancebecomes substantially infinitely large. Consequently, when the currentsiA, iB of the frequencies fA′, fB′ flow into the cylindrical portion22′, the current iA of the frequency fA′ is suppressed substantially tozero, while the current iB of the frequency fB′ is passed directlythrough the cylindrical portion 22′.

The current iB that has passed through the cylindrical portion 22′,however, is supplied to the cylindrical portion 23″ where it issuppressed substantially to zero.

In this way, according to this embodiment, even in the case where theunrequired radiation of different frequencies fA′, fB′ is generated inthe transmission path, the generation of the radiation of thesefrequencies can be effectively suppressed.

In the embodiments described above referring to the case, a coaxialcable is used as a transmission line. It is obvious, however, that thepresent invention is not limited to such a case.

INDUSTRIAL APPLICABILITY

As described above, in a low-EMI circuit board according to the presentinvention, a shield structure of high loss is formed of the low-EMIcircuit board. As a result, the radiation of differential mode can besuppressed and removed, while at the same time effectively suppressingand removing the radiation of common mode due to the resonance currentof high frequencies.

Also, the low-EMI cable connector according to the present invention,which is compact and light in weight and has a simple configuration, canhighly effectively suppress the current generating the unrequiredradiation from the cable, and therefore, as compared with theconventional version using a ferrite core, has a very great industrialeffect of application.

What is claimed is:
 1. A low-EMI cable connector mounted on atransmission cable for connecting units, comprising n (n: 1, 2, . . . )cylindrical members having a dielectric portion arranged on the innersurface thereof for surrounding the whole periphery of said transmissioncable, characterized in that a short-circuiting member for covering thewhole periphery of said transmission cable is arranged on thetermination side of said cylindrical member thereby to form ashort-circuiting termination line, and the resonance frequency of saidshort-circuiting termination line is equal to the resonance frequency ofsaid transmission cable.
 2. A low-EMI cable connector as described inclaim 1, characterized in that the length li of the i-th (i: 1, 2, . . ., n) one of said cylindrical members is given as $\begin{matrix}{1_{t} = {\frac{1}{4} \times \frac{\lambda_{i}}{\sqrt{ɛ_{ri}}}}} & \left( {{Expression}\quad 1} \right)\end{matrix}$

where λi=c/fi c=velocity of light fi=i-th fundamental resonancefrequency of said transmission cable, and εri=dielectric constant ofsaid dielectric portion of said i-th cylindrical member.
 3. A low-EMIcable connector as described in claim 1 or 2, characterized in that aresistor having a matching termination resistance value is arranged onthe termination side of said cylindrical member.
 4. A low-EMI cableconnector as described in claim 1 or 2, characterized in that aplurality of said cylindrical members are arranged coaxially.
 5. Alow-EMI cable connector as described in claim 1 or 2, characterized inthat said short-circuiting member is removably configured.
 6. A low-EMIcable connector as described in claim 1 or 2, characterized in that aplurality of said cylindrical members share the center axis and arearranged along said center axis.
 7. A low-EMI cable connector asdescribed in claim 1 or 2, characterized in that said cylindricalmembers are each adjustable in the position along the direction of thecenter axis.
 8. A low-EMI cable connector mounted on a transmissioncable for connecting units, comprising a cylindrical member with adielectric portion arranged on the inner surface thereof and coveringthe whole periphery of said transmission cable, characterized in that aresistor constituting a matching termination resistor is provided on thetermination side of said cylindrical member.
 9. A low-EMI cableconnector mounted on a transmission cable for connecting units,comprising n (n: 1, 2, . . . ) cylindrical members with a dielectricportion arranged on the inner surface thereof and covering the wholeperiphery of said transmission cable, characterized in that thetermination side of said cylindrical members is an open end constitutingan open termination line, and the resonance frequency of said opentermination line is equal to the resonance frequency of saidtransmission cable.
 10. A low-EMI cable connector as described in claim9, characterized in that the length li of the i-th (i: 1, 2, . . . , n)one of said cylindrical members is given as $\begin{matrix}{\frac{1_{t}}{2} = {\frac{1}{4} \times \frac{\lambda_{i}}{\sqrt{ɛ_{ri}}}}} & \left( {{Expression}\quad 2} \right)\end{matrix}$

where λi=c/fi c=velocity of light fi=i-th fundamental resonancefrequency of said transmission cable εri=dielectric constant of saiddielectric portion of said i-th cylindrical member.
 11. A low-EMI cableconnector as described in claim 9 or 10, characterized in that aplurality of said cylindrical members share the center axis and arearranged along said center axis.
 12. A low-EMI cable connector asdescribed in claim 9 or 10, characterized in that said cylindricalmembers are each adjustable in the position along the direction of thecenter axis.
 13. A low-EMI cable connector as described in any one ofclaims 2 and 10, characterized in that the dielectric constant of saiddielectric portion is equal to or larger than about
 300. 14. A low-EMIcable connector mounted on a transmission cable for connecting units,comprising: a first cylindrical member having a dielectric portionarranged on the inner surface thereof for surrounding the transmissioncable; a second cylindrical member having a dielectric portion arrangedon the inner surface thereof for disposition on the first cylindricalmember; a short-circuiting member for covering said transmission cableis arranged on the termination side of said first cylindrical memberthereby to form a first short-circuiting termination line; a secondshort-circuiting member for covering the first cylindrical member isarranged on the second cylindrical member thereby to form a secondshort-circuiting termination line; and a resonance frequency of saidfirst short-circuiting termination line and a resonance frequency ofsaid second short-circuiting termination line are different.
 15. Alow-EMI cable connector mounted on a transmission cable for connectingunits, comprising: a first cylindrical member having a dielectricportion arranged on the inner surface thereof and covering thetransmission cable; a second cylindrical member having a dielectricportion arranged on the inner surface thereof for disposition on thefirst cylindrical member; a termination side of said first cylindricalmember is an open end constituting a first open termination line; atermination side of said second cylindrical member is an open endconstituting a second open termination line; and a resonance frequencyof said first open termination line and a resonance frequency of saidsecond open termination line are different.